Coronary blood flow capacity is increased in hearts of exercise trained (EX) pigs due, in part to altered control of coronary vascular resistance. The goal of Project 3 is to elucidate mechanisms responsible for these training-induced alterations. Vasomotor responsiveness and endothelium-mediated control mechanisms will be examined in vitro in isolated coronary resistance arteries (near-resistance: 150 to 250 microm diameter, ID, and resistance arteries: 50 to 150 microm diameter, ID) mounted on glass micropipets and microvessel myographs. Research proposed for Aim 1 will determine whether mechanisms for training-induced increases in myogenic reactivity in isolated coronary resistance arteries include: a) increased responsiveness of microvascular endothelium to changes in intraluminal pressure, b) increased stretch-induced Ca2+ release and/or influx in vascular smooth muscle (VSM) cells, and/or c) enhanced stretch-induced increases in Ca2+ sensitivity of VSM contractile elements. The central hypothesis of Aims 2 and 3 is that enhanced endothelium-mediated vasodilator responses in the coronary microcirculation of EX pigs results from training-induced, increases in the constitutive form of nitric oxide synthase (ecNOS) in endothelium. Experiments proposed in Aim 2 use measurements of responses to endothelium-dependent vasodilator, substance-P, bradykinin, and intraluminal flow to determine whether endothelium-mediated vasodilation is enhanced throughout the coronary microcirculation after exercise training. Aim 3 will determine whether there is an increase in ecNOS in coronary resistance arteries from EX pigs. ecNOS activity will be measured in vitro, ecNOS content will be examined with Western blot analysis, and the distribution of ecNOS will be examined with immunohistochemistry. If ecNOS is not increased, we will test the hypothesis that enhanced endothelium-mediated vasodilator responses result from training-induced increases in antioxidants. Aim 4 is designed to determine the time course of training induced adaptations. Aim 5 will measure interactions between myogenic responses and flow- induced vasodilation in resistance arteries isolated from EX and SED pigs. Research designed for Aim 6 will determine whether endothelium- mediated vasodilation produces greater blood flow in the intact coronary circulation of EX pigs compared to SED. Completion of the research proposed in Project 3 will: A) elucidate cellular/molecular mechanisms that regulate VSM and endothelium in the coronary microcirculation, B) determine mechanisms responsible for training-induced changes in myogenic responses and endothelium-mediated vasoregulation in the coronary microcirculation, C) improve understanding of integration of blood flow control and vascular exchange in the coronary microcirculation., and D) Determine mechanisms for training-induced increases in coronary blood flow capacity. This research will provide a solid fundamental basis for understanding the effects of training on normal coronary vascular function. This understanding is essential to the development of concepts concerning interactions among exercise training, hyperlipidemia, and coronary disease in the coronary microcirculation.